This invention relates to the field of mechanisms capable of anchoring and electively releasing large loads from a retaining anchorage such as might be employed in controlling the parachute extraction of cargo from a moving aircraft.
The use of a parachute to extract cargo from the rear of a moving aircraft at either high or low altitudes has become an accepted procedure in military operations. The desirability of rapidly delivering relatively large quantities of military hardware with minimal exposure of the delivering aircraft to hostile military action and the absence of aircraft ground turnaround time at the cargo delivery site are two readily identified advantages of a parachute delivery arrangement.
The low level version of this parachute delivery arrangement, Low Altitude Parachute Extraction System, or LAPES, has become especially important in the era of brush fire or limited involvement conflicts where there is need to deliver cargo from supply centers located far from the delivery point. The C-5, C-130 and now the C-17 aircraft used by the U.S. Air Force have been especially adapted for achieving LAPES and also high altitude parachute cargo delivery. As cargo aircraft have evolved and refinements of the parachute delivery technique have occurred, the size and weight of the delivered cargo loads have increased notably until presently, this form of delivery for loads weighing as much as 55,000 pounds or 271/2 tons is contemplated.
Accompanying the military advantages of this form of cargo delivery, however, is a recognized set of operational hazards to the cargo, the delivering aircraft, the equipment and personnel in the delivery zone and the crewmembers of the delivering aircraft. One need only consider the consequences of a delivery mishap wherein cargo the size of a military truck or armored tank became temporarily jammed in the open rear aperture of a delivering aircraft or a large cargo extraction parachute became inflated and irremovably tethered to the aircraft--making the aircraft incapable of flying, in order to recognize the need for sure and positive equipment operation in LAPES delivery sequence.
An additional important consideration in the LAPES environment concerns need for positive and remotely controllable initiation of a LAPES event--often by the aircraft pilot, in order that the pilot be prepared for the significant changes in aircraft flight characteristics resulting from shifting and removal of heavy cargo loads with respect to the aircraft. Moving of a 55,000 pound load from a location forward of an aircraft center of gravity point to a location rearward of the aircraft center of gravity point and thence suddenly out of the aircraft can be expected to have noticeable influence on the aircraft flight characteristics. This influence is especially of interest to the aircraft pilot in the usual LAPES scenario where the aircraft is flying at altitudes as low as 10 feet above the ground. Removal of a cargo load of this magnitude from the rear of an aircraft can be expected, for example, to produce a sudden nose-down tendency in the aircraft flight path.
One of the better arrangements for accommodating such changes in a flying aircraft has been found to reside in providing the final release of cargo in response to an act executed by the aircraft pilot--a release accomplished without the intervention of a loadmaster or other "middleman" crewmembers. Pilot initiated cargo releases allow the pilot to be optimally prepared to compensate for flight path changes of the aircraft. Such cargo releases, without question call for highly reliable and remotely controllable cargo release apparatus.
Precise control of the cargo release event is of course also necessary in order that the cargo arrive in the intended location on the ground. For an aircraft moving at a speed of 120 miles per hour each one-tenth second of error in cargo release time corresponds to eighteen feet of error in the cargo delivery point. One need only consider the accounts of cargo delivery mishaps recorded in the WWII Normandy invasion history, including cargo falling into water or into enemy hands, to appreciate the military significance of precise air cargo delivery operations.
Some apparatus and procedures used for LAPES and other forms of air cargo delivery have been documented in the patent art; this art includes the patents of Leger et al, U.S. Pat. No. 4,398,686 and U.S. Pat. No. 3,670,999; Bolender et al, U.S. Pat. No. 4,303,213; Fielding et al, U.S. Pat No. 3,865,333; Hosterman et al, U.S. Pat. No. 3,801,051; an earlier patent of Leger, U.S. Pat. No. 3,670,999; Kriesel, U.S. Pat. No. 3,396,924; Cotton, U.S. Pat. No. 3,113,751; and Minty et al, U.S. Pat. No. 2,868,581. The disclosure of these patents is hereby incorporated by reference into the present specification.
The cargo delivering apparatus described in these patents can generally be considered to fall in two classes, i.e., arrangements achieving cargo extraction from the aircraft upon manual actuation of a cargo releasing towplate or tow/release apparatus--an actuation accomplished using flexible cables, lever arms, control rods, and similar linkages, and arrangements using electrical solenoid actuation of the towplate apparatus. Inherently, in early designs, it has been necessary to carefully limit the amount of friction involved in achieving a towplate release event in order that manual force or the force provided by a reasonably sized electrical solenoid be capable of operating the towplate mechanism. Since considerable loading of the towplate by a towed drogue parachute or other loads is usually involved, the incorporation of ball bearings and other low friction arrangements is usually necessary in these towplate mechanisms. Modern design standards, for example, call for a towplate release event to be within the capability of a fiftieth percentile female operator--an operator capable of providing 15 lbs. of normal exertion force and 30 lbs. of maximum force.
Numerous problems have developed with these previous towplate devices; such problems preclude the satisfactory operation of the towplate over long time periods especially operation that must be achieved with forces of limited magnitude. Included in these problems are bending and distortion of towplate elements in repeated use (the lever arm 71 in the 4,398,686 patent towplate has been especially troublesome in this regard), the increase in towplate operating force as a result of dirt fouling, corrosion, and the absence of satisfactory long-term lubrication for the employed bearings, frequent failure of the towplate solenoid member and the undesirability of an exposed towplate mechanism in most aircraft. Electrical solenoid failure has been particularly troublesome in prior towplate devices because such solenoids are necessarily made small in size and light in weight in comparison with the needed solenoid output force--operation near the limit of solenoid fabrication material capability is thus required. In this environment, eventual circumstances such as inexperienced operators, control switch malfunctions, or a jammed towplate requiring several release attempts are sufficient to extend solenoid materials such as the magnet wire windings over the threshold of failure.
A particularly notable limitation of prior patent towplate devices has been their usability only with forces in the range of zero to 7,000 lbs.; that is, with prior devices the force exerted by a drogue parachute trailing the cargo aircraft was limited to the range of 7,000 lbs. Drogue parachute forces an order of magnitude greater than this value or 70,000 lbs. are contemplated in the present invention apparatus.